In an electrical connector assembly, to prevent water from entering the electrical connector assembly, a rubber seal ring that is separate from a housing is interposed between a pair of electrical connectors, that is, between a male connector and a female connector mated with each other. In general, water falls onto an upper surface of the electrical connector assembly in the vertical direction. Accordingly, there is a high probability of the water received on the upper surface entering a gap between the male connector and the female connector. The seal ring allows the electrical connector assembly to be waterproof but requires the provision of an additional part in forming the electrical connector assembly. Alternatively, in applications in which the electrical connector assembly is placed at a position less likely to be exposed to water, it is possible to ensure a certain level of waterproof performance, or a moderate waterproof performance, without providing a seal ring.
International Patent Application Publication No. WO 2013/042714 proposes an electronic circuit device for airbag deployment that reliably drains water to a recessed portion formed in a casing, regardless of the position where liquid drops onto the electronic circuit device or the angle at which the electronic circuit device is attached to a vehicle. The electronic circuit device has an enlarged diameter portion 12c formed across the entire area in the circumferential direction of one side surface 12b of a casing 12, excluding a bottom portion 12a. The enlarged diameter portion 12c has an opening gradually expanding toward the outside of an opening end surface. The electronic circuit device has a recessed portion 12e formed between the one side surface 12b of the casing 12 and the other side surface 12d of the casing 12 disposed on a side opposite to the one side surface 12b. The recessed portion 12e forms a drainage groove, and has inclined surfaces which are inclined toward the bottom portion 12a. 
The electronic circuit device disclosed in WO 2013/042714 has the enlarged diameter portion 12c formed across the entire area in the circumferential direction excluding the bottom portion 12a, and the electronic circuit device consequently cannot be formed in a compact manner. Further, although the electronic circuit device disclosed in WO 2013/042714 is provided with a connector 13, a main portion of the connector 13 is accommodated in an accommodating chamber; it is not assumed that water falls onto a boundary portion between a pair of connectors.